The present invention relates to improvements in interference suppression systems and more particularly to side-lobe cancellers operating in a multiple signal environment.
Generally, interference suppression systems are designed to reduce the presence of undesired signals in a signal receiving system. As is known, in particular systems, such as radar systems, the characteristics of the receiving antenna are such that undesired signals which are received in the side-lobes interfere with the isolation of the target signal received in the main lobe. Accordingly, to isolate the main lobe signals, side-lobe cancellers have been used to cancel interference from the side-lobe of the main radar antenna as exampled by U.S. Pat. No. 3,202,990 to Paul W. Howells.
Conventionally, receiving systems employing side-lobe cancellers have used auxiliary receiving channels to sample the environment in which a desired signal may be present. Each auxiliary signal forms an input which is used by the canceller system in an attempt to eliminate undesired (interference) signals which may be present in conjunction with the desired signals is a main channel. In U.S. Pat. No. 3,202,990 each canceller loop attempts to decorrelate the main channel signal from its auxiliary input signal by first phase shifting and amplitude weighting the auxiliary input and then subtracting this modified signal from the main channel to reduce interference.
Such systems as described above work well to eliminate main channel interference with a single canceller loop when only one interference source is present. However, when multiple interference sources are involved, conventional systems have been ineffective in reducing interference even though multiple canceller loops have been used. In theory, if N independent interference sources are present forming a signal environment, the interference signals may be cancelled from the desired signal by multiple canceller loops fed by inputs from N separate auxiliary channels. In practice, however, it has been found that effective cancellation is limited by the relative independence of the auxiliary inputs and by the configuration of the auxiliary antennas relative to the main antenna.
While prior known techniques have in some cases been successful in improving cancellation by making the auxiliary inputs relatively independent of one another, the same have required additional structures which render the system more complex. Other techniques have used many auxiliary channels and canceller loops to provide more samples of the interference environment but still exhibit ineffective cancellation even where the number of auxiliary channels exceeds the number of interference sources. As a result, present systems employing a fixed configuration of auxiliary receiving channels fail to provide adequate protection against sophisticated multiple interference countermeasure techniques.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide a canceller system of improved performance and versatility in a multiple signal environment.